The invention relates generally to a clamping device, and more specifically to a self-tightening, easily releasable clamping device, is particularly well-suited for use in connection with sheet materials. For example, the present invention may be advantageously used with textile racking frames where a plurality of the clamps are arranged around the periphery the frame to hold the edges of a fabric or other textile workpiece during manufacturing.
Various types of clamps for with use with racking frames are known in the prior art. For example, in one type of fabric clamp illustrated in FIG. 1, an upper jaw 1 is pivotally connected to a lower jaw 2 by a pivot hinge rod 3. The lower jaw has a mounting flange 4 for attachment to the racking frame by screws or other suitable fasteners. A spring 5 is provided around the rod 3 to bias the upper jaw 1 into the closed position. In operation, the user or operator must press the handle part of the upper jaw 1 against the spring force to open the jaw for insertion of the fabric. Then, the operator releases the upper jaw to clamp the fabric by spring force. To remove the fabric, the upper jaw must be depressed against the spring force once again.
The prior art clamp shown in FIG. 1 suffers from several disadvantages. For example, because the operator must individually open each clamp, the clamp is somewhat cumbersome and time-consuming to operate on a rack that has a large number of such clamps. Also, the operator must apply pressure against the spring force each time the fabric is inserted, and must apply the pressure once again to release the fabric for removal. The cumulative manual effort required by the operator in opening and closing the clamps, therefore, can be quite significant, particularly for racking frames which include large numbers of clamps. Additionally, the gripping force applied to the fabric is limited by the relatively small contact area between the clamp and the fabric, and by the force that can be provided by the spring. If the spring force is too high, an operator will not be able to overcome it easily to release the clamp. However, if the spring force is too low, there may be insufficient gripping power to hold the fabric if it is pulled outwardly from the clamp.
Efforts have been made to increase the holding power of this type of clamp, particularly for fabrics such as chintz which have a slick finish. Another prior art fabric clamp is shown in FIG. 2. This clamp has an upper jaw 1', a lower jaw 2', a hinge pin 3', a frame mounting portion 4' and a spring 5' similar those of the clamp in FIG. 1. The clamp shown in FIG. 2 further includes two perforations 6' arranged in the lower jaw 2', and two mating protrusions 7' provided on the upper jaw 1'. As the clamp grips the fabric, the protrusions press the fabric through the perforations to enhance gripping power. However, although the perforations and protrusions of this clamp enhance the gripping power, this clamp also suffers from the deficiencies that it is difficult and time-consuming for the user to manually operate each clamp individually--once to insert the fabric and a second time to release it. Also, the user must apply a relatively large force to overcome the spring bias, and the gripping power remains limited by the spring force.
Accordingly, there is a need for a fabric clamp that is quickly and easily operated to release the fabric without the need for individual attention on the part of the operator and without the need to overcome a large spring force. There is also a need for a clamp, that, once released, stays in the open position to accept the insertion of fabric, and is easy to close onto the fabric without the need to overcome a large spring force. Another need is for a clamp that is self-tightening--i.e., a clamp in which the gripping force applied by the clamp increases when the fabric is pulled outwardly from the clamp. There is also a need for a clamp having the above advantages that can be used in a system where a plurality of clamps can be used on a racking frame. Finally there is a need for a method of holding a fabric or textile on a racking clamp that reduces operator time and effort as compared to the prior art clamps.